1. Field of the Invention
The present invention relates to an axial piston device such as a pump unit or a motor unit.
2. Related Art
An axial piston device comprising a cylinder block rotated about an axis and a piston accommodated in a slidable manner in an axial direction with respect to the cylinder block while being rotated about the axis together with the cylinder block has been widely utilized as a pump unit to be used as a hydraulic source with respect to hydraulic equipment such as a hydraulic motor or as a motor unit to be hydraulically driven by a hydraulic source such as a hydraulic pump.
Hereinafter, description will be given of a conventional axial piston device by way of a pump unit.
A conventional pump unit comprises, for example, a housing which has a housing body opened at a first end thereof and a plate attached to the first end of the housing body, a pump shaft which is supported by the housing and is driven by a drive source, and a pump body which is accommodated inside the housing and is rotatably driven by the pump shaft, wherein each of a discharge port and a suction port of the pump body is hydraulically connected in circulation to a corresponding hydraulic device such as a hydraulic motor.
That is to say, a pair of oil passages communicating with the discharge port and the suction port of the pump body, respectively, is formed at the plate. Thus, pressurized oil is supplied from the pump body to the hydraulic device via one of the oil passages, and further, return oil is returned to the pump body from the hydraulic device via the other one of the oil passages.
In the pump unit after assembly, air is mixed inside the pair of oil passages; therefore, the pair of oil passages is required to be deaerated.
In other words, the pump unit and the hydraulic device are connected via the pair of oil passages, thereby forming a circulation circuit, wherein the circulation circuit is required to be sufficiently deaerated upon filling oil into the circulation circuit.
In regard to this point, in the conventional pump unit, a drain oil passage for allowing the pair of oil passages to communicate with an oil sump is formed at the plate, and further, a shutoff valve is disposed inside the drain oil passage in such a manner as to be positionally adjusted in an axial direction (see U.S. Pat. No. 6,332,393).
In particular, a valve seat is provided at the drain oil passage. The position of the shutoff valve in the axial direction can be adjusted in such a manner that the shutoff valve can take a shutoff position at which the shutoff valve is in contact with the valve seat so as to have the drain oil passage shut off and a communication position at which the shutoff valve is apart from the valve seat in the axial direction so as to have the drain oil passage communicated.
In this conventional pump unit, the pair of oil passages can communicate with or can be cut out of the oil sump by operating the shutoff valve, with an attendant problem of impossibility of speedy switching between the communication and shutoff.
Namely, in the conventional pump unit, the position of the shutoff valve in the axial direction can be adjusted with respect to the plate owing to screw connection. Consequently, in order to move the shutoff valve from the shutoff position to the communication position at which a sufficient opening width is secured, the shutoff valve must be rotated on an axis many times.
The present invention has been accomplished in view of the above prior art. Therefore, a primary object of the present invention is to provide an axial piston device in which an oil passage can be securely and speedily deaerated.